Wow, a few of those guys are awesome. http://en.wikipedia.org/wiki/William_Ash_(writer) who renounced because the US had a stance of neutrality vs. the Nazis for too long, so he renounced to become a Canadian military pilot. Awarded MBE.

Apparently they used to be a lot more "automatic" about revoking citizenship for acts (like joining a foreign military as an officer) where there wasn't clear intent to renounce citizenship. It now requires a seriously affirmative act; essentially showing up at a consulate and making a declaration, demonstrating that it's intentional and informed, etc.